The 1.50 isn't that much of an oddball. The 1.32 is. The 1.65 from a 165 is pretty odd as it has the short input yoke. Only other odd thing, and it is no longer an issue, was on the old drives with the 1.65 you had to mark the gear mesh when resealing the upper as it was the same tooth count so they always ran the same pattern. Merc later dropped this from the service manual. The old gear sets had arrows on them if I recall from dad.Nice. This thread comes up in google searches for the subject, so at least this information is out there now (although Gemini still gets it wrong)
Also interesting that the 233 uses a rare ratio. And I thought my 1.50 ratio was a needle in a haystack... good thing I don't have that!
Sounds like more testing with the 21 pitch may be warranted. 100 under minimum ain't much... Maybe should have gone with 20... 19 too low? We shall see. But 19 was cheap (to the tune of $110 shipped for 5 blades that are the correct length and fit my existing 3-blade hub), so if it's wrong I'm not out a super large wad of moolah.
Coincidentally most engines run better the higher the gear ratio you use, it just puts you out of normal prop range most of the time. Looking around in threads here you see most people end up in the 17p to 23p range with 19 and 21 being very common. Granted we're getting into the racing end of the weeds now... but spinning a higher pitch slower is more efficient.... and would never be a noticeable difference until we're hitting double the speeds we're talking about right now.